The present invention relates to the field of conveyor technology and concerns a conveyor system comprising a large number of conveyor elements movable along predetermined conveyor paths as well as at least one stationarily arranged drive means acting locally on the conveyor elements in an accelerating or braking manner.
The conveyor elements are connected to one another at constant or variable distances via jointed or flexible connections to form a chain or they are displaceable independently of one another along the conveyor paths and are, where necessary or advantageous, guided by suitable guide means. Such guide means are, for example, guide channels or channel-like rail arrangements along which the conveyor elements roll or slide. Chain wheels, which are locally in engagement with the conveyor elements, may also serve as guide means.
The at least one drive means acts locally in an accelerating or braking manner on those conveyor elements that are present in the effective range of the drive means (drive region) and which, where appropriate, transmit the accelerating or braking effect by pulling or pushing to other conveyor elements.
Conveyor systems of the mentioned type are, for example, used for piece good processing where large numbers of equal or similar objects are processed in a plurality of processing steps. The objects are, for example, individually held by grippers, each gripper being arranged on one conveyor element. For a process or a part thereof, in which all objects run through the same sequence of processing steps one after the other in a uniformly cycled manner, the conveyor elements are advantageously connected with one another to form a chain. For a process or a part thereof in which the objects run through individual sequences of processing steps and/or are individually processed in processing stations, and therefore the conveying course of individual objects may be different from one another, conveyor elements being independent of one another are advantageous as they allow conveyance without the need of repeated transfer of the objects from one gripper to another one.
Drives for conveyor systems of the mentioned type are, for example, revolving gearwheels or toothed belts which, for transmitting a drive force, are brought into engagement with the conveyor elements in a positive fit and essentially without slip. Through such engagement independent conveyor elements lose their independence, since all conveyor elements in engagement are compellingly subjected to the conveying speed and the conveying cycle of the drive. Particularly in the region in which the conveyor elements come into engagement with the drive, this entails jolt-like movements through which high forces act on the material and lead to an increased wear.
So-called string drives, as for example described in the publication EP-0936161, are also applicable in the mentioned conveyor systems. Such a drive comprises a driven string being moved parallel to the conveyor path of the conveyor elements and, for being driven, the conveyor elements are coupled to the string via suitable means essentially effecting a friction fit. Since the string in contrast to gearwheels or toothed belts represents a so-to-say continuous coupling means, the conveyor elements may be coupled to it independent of a conveyor cycle. However, as slip between the string and conveyor elements increases material wear, slip must be avoided as much as possible. Therefore, in such a system the drive speed or the speed of the string respectively dictates the conveying speed. This means that although the conveyor elements can be driven in a manner independent of a conveyor cycle, i.e., at any distance to one another, once coupled to the string they can only be driven at the speed of the string. This leads to the disadvantages mentioned hereinbefore in connection with the toothed drive means.
From the publication WO-99/33731 it is also known to drive the conveyor elements of a conveyor system by coupling them to drive elements with the help of a magnetic attraction force acting essentially transverse to the conveyor path and by moving the drive elements in a drive region with the help of gearwheels or similar means parallel to the conveyor path of the conveyor elements. If the drive means and the parts of the conveyor elements to be coupled thereto are designed suitably flat, such a magnetic coupling allows conveyance without a predetermined conveying cycle and, since slip between conveyor elements and drive elements entails lower material wear than this is the case with the above mentioned string drive, there is no compelling predetermined conveying speed for the conveyor elements. With such a drive it is, for example, possible to bring conveyor elements into the effective region of the drive in a jolt-free manner or to push conveyor elements into a buffer having a variable length in which buffer the conveyor elements have a conveyor speed different from the drive speed or even stand still.
It is an object of the invention to provide a conveyor system of the above mentioned type, in which conveyor system conveyor elements displaceable along conveyor paths are accelerated or decelerated in a drive region by at least one essentially stationary drive means such that neither the conveying cycle nor the conveying speed of the conveyor elements are compellingly predetermined by the drive. All the same, the drive is to be simple with regard to the device and control and is to function essentially without material wear.
The drive means of the conveyor system according to the invention works with the eddy current principle. Eddy currents are produced in a conducting material by way of a magnetic field changing with time. Thereby forces counteracting the change are affected. This effect is exploited in the drive means of the conveyor system according to the invention in that a magnetic field changing with time is produced in a conducting part by a relative movement between this conducting part and a magnetic field which changes locally in the direction of the relative movement (field lines with components transverse to the direction of the relative movement). The eddy currents produced in the conducting part effect forces counteracting the relative movement, i.e. to decelerate or brake it.
In the conveyor system according to the invention, the conveyor elements comprise conducting parts and the drive means comprise a row of magnetic elements (elements producing a magnetic field) for producing a locally changing magnetic field in a drive region. The row is aligned at least partly substantially parallel to the conveyor path. The magnet element row of the drive means and the conducting parts of the conveyor elements are arranged relative to one another such that the conducting parts of conveyor elements present in the drive region are positioned in the effective range of the magnet element row. The relative movement necessary for producing the eddy currents is achieved by moving the conveyor elements along the conveyor path through the drive region and/or by moving the magnet element row or parts thereof essentially parallel to the conveyor path.
In the magnet row, magnet elements producing a magnetic field alternate with elements producing another magnetic field or with elements producing no magnetic field. The elements producing the various magnetic fields are, for example, permanent-magnetic north and south poles, which in the row are arranged alternating next to one another.
The magnitude of the braking or accelerating effect between the drive means and the conveyor elements is dependent on the magnitude of the temporal change of the magnetic field, i.e. on the strength of the magnetic fields produced by the magnet elements, on the differences between the magnetic fields produced by the magnet elements of various types, on the distance between the magnet elements in the magnet element row, on the relative speed between the magnet element row and conveyor elements and on the distance between the conducting parts and the conveyor elements. A given drive effects braking or accelerating forces, which are essentially proportional to the speed of the relative movement and inversely proportional to the square of the distance between the magnet element row and the conducting parts.
If the magnet element row is arranged stationary, it can only act on the conveyor elements in a braking manner. Similarly, a magnet element row being moved essentially parallel to the conveyor path in the direction opposite to the conveying direction can only act on the conveyor elements in a braking manner. If the magnet element row is moved essentially parallel to the conveyor path and in the conveying direction, it decelerates conveyor elements having a speed larger than the speed of the magnet element row and it accelerates conveyor elements having a smaller speed.
A preferred embodiment of the conveyor system according to the invention comprises a drive means comprising an annular row of alternatingly arranged permanent magnets such that south and north poles of the magnets face the conducting parts alternatingly. The permanent magnets are arranged with the smallest possible distance to one another, for example, on a rotating disk or a revolving conveyor means. The disk or deflection means of the conveyor means respectively rotate about axes which are arranged essentially transverse to the conveyor path and spaced from the conveyor path such that the part of the magnet element row lying closest to the conveyor path has a revolving direction parallel to the conveyor path or has a tangent parallel to the conveyor path.
In order to avoid accelerations of the conveyor elements transverse to the conveyor path as much as possible, it is advantageous to arrange in a drive region two equal rows of magnet elements, with the rows lying as exactly as possible opposite to one another and revolving with the same speeds in opposite directions.
The conducting parts of the conveyor elements consist advantageously of a paramagnetic material, such as aluminum, and project transversely to the conveying direction into the effective range of the magnet element row, wherein the distance between conducting parts and magnet elements is as small as possible. The conducting parts are designed such that between conducting parts of neighboring conveyor elements there is no conducting connection, even when the conveyor elements move in a compact succession through the drive region.
For controlling the accelerating or braking effect of the drive means, the revolving speed of the magnet element row and/or the distance between magnet elements and conducting parts are adjustable. When using an electrically produced magnetic field it is also possible to adjust the strength of this field.